This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. To assess the validity of the raft hypothesis for the cellular plasma membrane we examined both the dynamics and equilibria of liquid-ordered (Lo) and liquid-disordered (Ld) phases in the brain-sphingomyelin (SPM) /dioleoylphosphatidylcholine (DOPC)/cholesterol model membrane system. Using the known phase diagram for this lipid system containing the phase boundary between the one-phase regions and the two-phase coexistence region of the Lo and Ld phases and an ESR spectral fitting method (New Method for Determining Tie-lines in Coexisting Membrane Phases using Spin-label ESR, Y.-W. Chiang, J. Zhao, J. Wu, Y. Shimoyama, G.W. Feigenson, J.H. Freed, Biophysica-Biochimica Acta 1668, 99-105,2005) from the linear superposition of boundary spectra, we determined one tieline thru the Lo+Ld coexistence region. Many pairs of Lo and Ld phase compositions can coexist at equilibrium for this ternary lipid system, so tielines are necessary to model rafts because they tell us which phase compositions coexist and therefore which pair of compositions best model the plasma membrane. If the Lo and Ld phases observed in the SPM/DOPC/cholesterol model system are physically similar to raft domains, then the dynamic parameters of spin probes in the model membranes determined from ESR spectral fitting should be similar to those of the same probe in plasma membranes. This was found to be true in that dynamic parameters from chain-labeled spin probes at the boundary compositions of the one tieline so far determined compared very well with dynamic parameters from the same spin probes in live cells (Coexisting Domains in the Plasma Membranes of Live Cells Characterized by ESR Spectroscopy, M.J. Swamy, L. Ciani, M. Ge, A.K. Smith, D. Holowka, B. Baird, and J.H. Freed, Biophys. J. 90, 4452-4465, 2006). A more extensive analysis of Lo and Ld phase equilibria and dynamic comparison between model and real plasma membranes will require the determination of the entire set of tielines for the Lo+Ld phase coexistence region.